In recent years, according to the demand for energy conservation of an image forming apparatus by the electrophotographic method, low-temperature fixing for fixing at a lower temperature is in progress, in order to reduce the consumption energy of the fixing device which consumes the largest power in the image forming apparatus.
To realize the low-temperature fixing, it is necessary to fuse a binder resin and a release agent in a toner at a low fixing temperature. For that purpose, generally, as a binder resin and a release agent (wax) in the toner, it may be considered to use ones having a low melting viscosity.
Furthermore, to obtain a toner suitable for a lower fixing temperature, it is necessary to use a release agent having a lower melting point Accordingly, a toner suitable for low-temperature fixing using a release agent having a low-melting point (hereinafter, also referred to as a “low melting point release agent”) has been proposed (for example, refer to Patent Documents 1 and 2).
Furthermore, from the viewpoint of the global environment preservation, reduction in energy consumption by the image forming apparatus is required, and in addition to development of the aforementioned toner suitable for low-temperature fixing, improvement of the fixing efficiency of the fixing device has been studied. As one of such fixing devices, in place of the conventional fixing device using a combination of a heating roller and a pressing roller, a fixing device having a belt-type heating body and a belt-type pressing body up and down has been proposed (for example, refer to Patent Documents 3 and 4).
This fixing device employs belt-type members in the heating and pressing section, whereby the width of the nip at the fixing part is broadened, and the fixing efficiency is improved.
However, when fixing is carried out using the proposed toner for low-temperature fixing in a high speed image forming apparatus (for example, an image forming apparatus of 40 sheets/minute with lateral direction feeding of A4 sized sheets) with a contact-heating fixing devise having a paired-belt constitution made up of a belt-type heating body and a belt-type pressing body mounted, problems arise that belt-shaped or stripe-shaped image defects are found on a fixed image and when obtained prints are piled up and left for a while, a document offset tends to occur.
When a toner image is fixed at a lower fixing temperature in order to solve the aforementioned problems, the belt-shaped image defect or stripe-shaped image defect is suppressed, however, the fixing property (fixing strength) is lowered.
Development of an image forming apparatus is desired, in which belt-shaped or stripe-shaped image defects are suppressed, print images having enough fixing strength are obtained and only limited print offset occurs after storing piled print images, even when fixing is carried out at a lower temperature in a high-speed image forming apparatus containing a contact-heating fixing devise having a paired-belt constitution.    Patent Document 1: Japanese Patent Application Publication Open to Public Inspection (hereafter referred to as JP-A) No. 2000-321815    Patent Document 2: JP-A No. 2000-275908    Patent Document 3: JP-A No. 7-140815    Patent Document 4: JP-A No. 2002-365948